THIS IS A RUSH TRANSCRIPT. THIS COPY MAY NOT BE IN ITS FINAL FORM AND MAY BE UPDATED.

[18:00:00]

WOLF BLITZER, CNN ANCHOR: Breaking news: seal of approval. President Obama endorses Hillary Clinton moments after meeting with Bernie Sanders at the White House. President Obama and Hillary Clinton now scheduled to hit the campaign trail together. Can they unify a divided Democratic Party?

"Delete your account." Hillary Clinton fires back at Donald Trump on social media after he slams her presidential endorsement, their exchange foreshadowing what's expected to be a very nasty general election fight. Can Clinton win a Twitter war with Trump?

And calming the storm. Trump meets with top donors and the GOP chairman as he tries to ease growing anxiety among Republicans, but the never Trump movement is once again gaining some strength. Are party leaders gathering a new strategy to deny Trump the nomination?

We want to welcome our viewers in the United States and around the world. I'm Wolf Blitzer. You're in THE SITUATION ROOM.

ANNOUNCER: This is CNN breaking news.

BLITZER: The breaking news, powerful backing from a one-time rival, President Obama making a hearty endorsement of Hillary Clinton. The Clinton campaign releasing a video of President Obama offering his support, saying, and I am quoting now, "I'm with her, I'm fired up and I can't wait to campaign with her."

The endorsement came just hours after the president met with Bernie Sanders at the White House in what was described as a very good and positive meeting.

Afterward, Sanders said he will continue to compete in next Tuesday's Washington, D.C., primary, but he also said he will work closely with Clinton to stop Donald Trump from becoming president.

The presumptive GOP nominee took to Twitter to slam President Obama's endorsement of Clinton. Trump is now working to reassure Republicans unnerved by his controversy-plagued campaign. He met with leading party donors and officials today, as he faces continued criticism for the racial attacks on a judge in a Trump University lawsuit. We're covering all of that, much more this hour with our guests,

He has been eager to jump into this race from the very start and start going after Donald Trump, and now he will do that. Now, Bernie Sanders just left the vice president's residence a few moments ago after meeting with him privately as well, on a day when Washington Democrats fell into line to focus on the battle ahead.

(BEGIN VIDEOTAPE)

OBAMA: I want to congratulate Hillary Clinton on making history.

ZELENY (voice-over): On the sidelines no more, President Obama offering a full-throated endorsement tonight of Hillary Clinton.

OBAMA: In fact, I don't think there's ever been someone so qualified to hold this office. And I'm with her. I am fired up and I cannot wait to get out there and campaign for Hillary.

ZELENY: Hillary Clinton welcoming the news on Twitter, writing: "Honored to have you with me, POTUS. I'm fired up and ready to go."

Democrats falling in line quickly, united around the idea of stopping Donald Trump. The president's blessing coming just hours after meeting with Bernie Sanders today in the Oval Office. The visit included a walk along the White House Colonnade, a courtesy normally afforded to visiting heads of state.

SEN. BERNIE SANDERS (VT-I), PRESIDENTIAL CANDIDATE: Let me begin by thanking President Obama and thanking Vice President Biden for the degree of impartiality they established during the course of this entire process.

UNIDENTIFIED MALE: Senator, welcome back.

ZELENY: Sanders also returning to a place Democratic leaders want him, back on Capitol Hill, meeting with Senate Democratic Leader Harry Reid and others.

SEN. HARRY REID (D-NV), MINORITY LEADER: I don't think Bernie Sanders is holding out for anything. He's -- I think he is somebody who is interested in changing the direction of the country. ZELENY: It was a daylong sign of respect and leverage for Sanders

after winning 22 states and aggressively challenging Clinton. Sanders didn't directly address plans to suspend his campaign, but did signal he is ready to unite Democrats against the presumptive Republican nominee.

SANDERS: Needless to say, I am going to do everything in my power and I will work as hard as I can to make sure that Donald Trump doesn't become president of the United States.

ZELENY: Trump also taking it all in, tweeting: "Obama just endorsed crooked Hillary. He wants four more years of Obama, but nobody else does."

Clinton firing back, "Delete your account."

Rivals for a year, Sanders and Clinton will soon come together.

SANDERS: I look forward to meeting with her in the near future.

ZELENY: And Clinton wants and needs his help, particularly firing up voters as he did across the country.

HILLARY RODHAM CLINTON (D), PRESIDENTIAL CANDIDATE: I'm looking forward to working with him to achieve our common goal, which is to defeat Donald Trump. And Senator Sanders has said he will work every day, every week to see that happen.

(END VIDEOTAPE)

[18:05:15]

ZELENY: And, tonight, Wolf, Elizabeth Warren will also add her voice to this growing chorus of Democrats supporting Hillary Clinton.

The Massachusetts senator has also been on the sidelines throughout this whole race and is eager to jump in. She's already aggressively attacking Donald Trump and will do so again tonight in a speech here in Washington.

Now, Sanders has stopped short of an endorsement himself, but he is likely to meet with his former rival soon and get behind the Clinton candidacy. The question in all this is whether his supporters who have spent the last year behind him against Clinton will take their cues from Sanders, Wolf, or go their own way.

BLITZER: Great question, an important one indeed. All right, thanks very much, Jeff Zeleny.

Donald Trump reacted to President Obama's endorsement of Hillary Clinton almost immediately, mocking it on Twitter.

CNN's Phil Mattingly is covering the Trump campaign for us.

Phil, there's still a lot of anxiety among top Republicans about Trump. What's the latest? PHIL MATTINGLY, CNN CORRESPONDENT: Well, no question.

As the Democratic Party very clearly is moving quickly to come together, the GOP still facing struggles. And, Wolf, it is a tension that goes beyond just Capitol Hill. It is governors, it's state lawmakers, it's donors, it is a party that desperately wants to shift into general election fight against Hillary Clinton, if only their candidate will let them.

(BEGIN VIDEOTAPE)

MATTINGLY (voice-over): Donald Trump today huddling with top donors and RNC Chairman Reince Priebus, part of his effort to quell growing unease inside the GOP.

DONALD TRUMP (R), PRESIDENTIAL CANDIDATE: I understand the responsibility of carrying the mantle.

MATTINGLY: Even as some top Republicans remain on the fence in the wake of Trump's attacks over the ethnicity of a judge presiding over a class-action lawsuit related to the now-defunct Trump University.

GOV. SCOTT WALKER (R), WISCONSIN: I hope he renounces what he said. I will be watching, as others are, closely. We will wait and see what happens going forward.

MATTINGLY: Ohio Governor John Kasich still wary of throwing his support behind Trump.

QUESTION: You don't even sound like you're on the fence.

GOV. JOHN KASICH (R), OHIO: No, I'm giving him a chance.

QUESTION: You are?

KASICH: Well, I mean, look...

QUESTION: It doesn't sound like it.

KASICH: He is trending the wrong way with me.

MATTINGLY: House Speaker Paul Ryan is again voicing frustration with Trump's actions, even as he sticks by the presumptive GOP nominee.

REP. PAUL RYAN (R-WI), SPEAKER OF THE HOUSE: Do I think that these kinds of antics are distracting and give us a campaign that we cannot be proud of? Yes. I have spoken very clearly about it. But I think and hope and believe that he can fix this, to the point where he can hopefully run a campaign that we can all be proud of.

MATTINGLY: Trump is facing very real concern over his ability to launch an effective general election campaign, and whether he can unite the party.

REP. LUKE MESSER (R), INDIANA: Our party is united that we are not for Hillary Clinton. The question is, can folks put on a Donald Trump T-shirt? And the reality is, it is hard to do that given this current rhetoric.

MATTINGLY: Once again leaving some in the party to consider whether avenues exist for a Trump alternative, such as unbinding delegates on the convention floor in July, an unlikely, yet now discussed possibility. The talk is only expected to escalate in the coming days, as top GOP lawmakers and donors meet at a finance retreat organized by 2012 GOP presidential nominee Mitt Romney, a vocal Trump critic.

MITT ROMNEY (R), FORMER PRESIDENTIAL CANDIDATE: There's plenty of evidence that Mr. Trump is a con man, a fake.

MATTINGLY: Trump allies briefing supporters on Capitol Hill today as the billionaire attempts to steady his campaign after a rocky last few days.

NEWT GINGRICH (R), FORMER SPEAKER OF THE HOUSE: Trump is learning how to be a candidate. Beat 16 pretty competent people, and now he is having to learn how to be a general election candidate.

MATTINGLY: All this as Republican donors, long spurned by Trump in the primary, are now voicing concerns of a financial shortfall that will be, at least according to one prominent donor, in the hundreds of millions of dollars when compared to Hillary Clinton's operation.

Trump moved to calm those anxieties in New York today, meeting with more than 60 top donors, his team laying out an aggressive fund- raising and finance strategy, aides say, all for the weeks ahead.

(END VIDEOTAPE)

MATTINGLY: And, Wolf, the donor worries are really only exacerbated by what they see on the other side.

The Clinton campaign, it has a finance network that has been tended to over decades, an organization that's been on the ground for months, and a team that has already raised nearly $200 million and has more than $30 million in the bank, the Trump campaign just over $2 million on hand.

Still, Trump has made clear he thinks he is playing by different rules, Wolf, and the shortfall or not, his presence alone will help make up some of those gaps. His chief strategist, Paul Manafort, walking out of that meeting today with donors, put it rather simply. "We will have enough to win" -- Wolf.

BLITZER: All right, thanks very much, Phil Mattingly.

Let's get some more on all of this.

Joining us now, the chairman, chairman, of the Hillary for America Campaign, the former White House chief of staff to Bill Clinton John Podesta.

[18:10:02]

John, thanks very much for joining us.

JOHN PODESTA, CLINTON CAMPAIGN CHAIRMAN: Nice to be with you, Wolf.

BLITZER: Now that you have the endorsement from the president, how do you go about capturing all those millions of Bernie Sanders supporters, especially those young people?

PODESTA: Well, look, we're going to go -- go out, and try to campaign, and earn their votes and earn their trust and welcome them into this campaign.

Bernie Sanders ran an extraordinary campaign. He reached out to people. But we're going to go starting next week on the road, talking about how we bring this country together, and why it is stronger together.

And Secretary Clinton will be joined by President Obama next Wednesday in Wisconsin. That's part of the process of reaching out to people, saying that she's got the right ideas, the values, the history to get the job done, to build an economy that's going to work for more people to be inclusive, to give people the chance to live up to their God- given potential.

So, we have got work to do, but we are on to a very good start. We are really excited about the president's endorsement, because he knows her so well. He knows that she can do the job.

BLITZER: He was very enthusiastic in that video, as we saw.

Sanders outside the White House today after his one-hour meeting with the president in the Oval Office saying he would be competing in next Tuesday's Washington, D.C., primary, he is not giving up yet.

Are you concerned that the longer he stays in the race, the less inclined his supporters will be to vote for Hillary Clinton in November?

PODESTA: Well, look, we have never tried to say he should stop campaigning. We respect his right to compete in the District of Columbia. He said he wants to make the case, and I think that Hillary joins him in that in wanting to see D.C. become a State of the Union.

But the fact is that she's the presumptive nominee. She called him on Tuesday night, suggested they sit down. He agreed to that. And as he said today, he wants to talk to her about how we take the fight, beat Donald Trump, and how we build a more inclusive economy that's going to, you know, take on special interests that have rigged the economy, so that working people can't succeed.

They agree on much of the agenda. So, I think it is time for them to get together and talk about how they can be partners in moving that forward and getting results the American people.

BLITZER: We are showing our viewers, John, some live pictures of a Bernie Sanders rally that's scheduled to get under way fairly soon in Washington, D.C. You see some of those supporters, a lot of young people over there.

We have also seen a lot of anger, a lot of vitriol from some of those Bernie Sanders supporters. Previously, Senator Sanders has said it was on Clinton's, Hillary Clinton's shoulders to unite the party.

What is Hillary Clinton prepared to do in order to embrace Bernie Sanders as she moves toward the general election, specifically at the convention?

PODESTA: Well, you know, Wolf, specifically, what she did was to call him Tuesday to congratulate him on a strong campaign, to suggest that they sit down and see whether they can join forces.

We have got a lot to work out. The platform committee met yesterday and today. But when you think about what they're both for, whether it's equal pay for women, more child care, affordable college, raising the minimum wage, they're quite close together really the issues.

And you compare that to the gulf that exists with Donald Trump, who thinks pay is too high in America, wants to punish women for exercising their reproductive rights, thinks climate change is a Chinese hoax, you know, I think they have a lot more in common than they have division.

Obviously, when you're in a primary campaign, you accentuate the differences, but what they both want to see is an economy that is working for working people. And I think when they sit down and talk that through, we will be able to figure out, have a great convention, bring the party together, and convince some of Senator Sanders' supporters who might be skeptical that she's moving in a strong direction, a progressive direction, she will get the job done as president of the United States.

And, most importantly, we have got to keep Donald Trump out of the Oval Office.

BLITZER: I understand. I want to just make sure that there are high- level discussions going on between the Bernie Sanders campaign and your campaign.

The hope from your side is to have a meeting. When is that meeting going to take place, do you know, between Senator Sanders and Hillary Clinton?

PODESTA: Well, we're -- yes, nothing has been scheduled yet, but we're -- I think both sides want to make sure that that happens and it happens relatively soon.

So, we are in discussions about that, and it really is a matter of aligning their schedules. Obviously, they're both busy people. So, but we're trying to get it done and get it done quick.

BLITZER: Following a meeting, assuming it will happen fairly soon, do you expect Bernie Sanders to do what President Obama is about to do, go out on the campaign trail with Hillary Clinton?

[18:15:05]

PODESTA: Well, you know, I think that we would welcome that.

And I think if you go back to 2008, after it became clear that Senator Obama had won the pledged delegates, was going to be the nominee of the Democratic Party, that's exactly what Hillary did. She went out even after that very hard-fought campaign. She endorsed him. She asked that his name be put into nomination by acclamation. She campaigned with him.

She campaigned separately. She raised money for him. I think there's a lot to be -- you know, this is a tough time. We have kind of been there. I know it's hard for Senator Sanders and his -- and Jane and for his team, but we hope that we can get his full support and that we will do everything we can, again, to team up these two extraordinary efforts to go and not just beat Trump, but to move the country forward, to get the job done that needs to be done for the American people, so that they can see their wages rise and ensure that everybody can lead a good life if they work hard and play by the rules.

BLITZER: Clearly, you hope he follows the Hillary Clinton playbook. What she did for then Senator Barack Obama eight years ago, you hope he does for her this time around.

All right, John, stand by. We have more to discuss, including a new phase. There's a real Twitter war under way right now between Hillary Clinton and Donald Trump. We will update our viewers. Much more right after this.

(COMMERCIAL BREAK)

[18:20:55]

BLITZER: The breaking news tonight, President Obama's endorsement of Hillary Clinton for president.

President Obama saying -- and I'm quoting him now -- "I don't think there's ever, ever been someone so qualified to hold this office."

We are back with the Clinton campaign chairman, John Podesta.

John, Donald Trump and Hillary Clinton, they have had a war on Twitter today. He called her crooked and that Obama just wants four more years of himself.

She then tweeted -- quote -- "Delete your account," to which respond -- and I will read to you what he said on Twitter -- "How long did it take your staff of 823 people to think that up? And where are your 33,000 e-mails that you deleted?"

So, she really seemed to walk into that one.

But is this campaign going to come down to a Twitter war?

PODESTA: Well, "Delete your account" is a joke on Twitter. And I think we were having some fun with him.

But he's one guy who probably should take that advice, because, you know, he tries to take everything down, tear everybody down, run things into the ground. And, as she said, she's going to take him on, but she's going to take him on, on a plane in which she uses his own words, as she did in San Diego on national security, will continue to do that, to show that he really just doesn't -- he's unfit for the office.

He doesn't have the character. He doesn't have the temperament. He doesn't have the ability, the depth, the knowledge to really lead this country. And I think, when people stop and think about Donald Trump in the Oval Office and the fact that his main skill is being able to dash off nasty tweets, I think they're going to think twice.

And that's why he is having so much trouble consolidating the Republican Party, and even the people who kind of took a deep gulp and endorsed him are now having second thoughts.

BLITZER: As you know, a source close to Senator Elizabeth Warren of Massachusetts has confirmed to CNN that she will endorse Hillary Clinton later today. We're standing by for that.

The RNC, the Republican National Committee, in the meantime, has weighed in with a statement. I will read it to you.

So, I guess the question is, Hillary Clinton is very excited, wants this endorsement from Elizabeth Warren, but what's your reaction to this charge from the Republican National Committee?

PODESTA: Look, look, you know, first of all, we very much are looking forward to support from Senator Warren. I am going to let her speak for herself.

But Hillary and the senator have been talking over the last couple of weeks. And when you look at the Wall Street plans that have been put down on the table, as independent experts have noted, Hillary has got the toughest plan. She particularly has pledged to protect the Consumer Finance Protection Board, which is so near and dear to Senator Warren's heart, because she helped create that whole institution.

And I think that, when you look at these -- Republican attack machine, over and over again, they level false charges. If you look at where fossil fuel is money lined up, it apparently is lined up behind Donald Trump. At least, that's what it looked like when he was out in North Dakota saying that he would do anything to support them, whereas she has pointed to the need to move to a clean energy future.

So, they will continue to, you know, attack us, throw out charges, but we are going to run a campaign that is focused on moving the country forward, trying to bring the country together, arguing against the division, the hatred, the bigotry, as we saw last week, when Donald Trump attacked Judge Curiel.

We are going to try to bring the country together, and give a positive foundation for building an economy that's going to work for the American people.

[18:25:02]

So, we're used to it. They have been doing it for a long time. And I'm sure that that's their playbook. So, they will continue to do it.

BLITZER: All right.

PODESTA: But what we need to do is to expose Donald Trump for who he really is, which is basically a fraud and unfit to be in the office, and to project what she wants to do for the American people.

BLITZER: All right, John Podesta, the campaign chairman for the Hillary Clinton campaign, John, thanks very much for joining us.

PODESTA: Thanks, Wolf.

BLITZER: Tomorrow, I will be sitting down with the 2012 Republican presidential nominee, Mitt Romney, to discuss, among other things, his opposition to Donald Trump, his fund-raising efforts for the Republican Party.

This is an exclusive interview. You won't want to miss it, tomorrow, 5:00 p.m. Eastern right here in THE SITUATION ROOM.

Coming up, the breaking news continuing: Will major endorsements, including President Obama's, help Hillary Clinton unify the divided Democratic Party?

Plus, the growing unease among Republicans over Donald Trump. Is a new effort under way to deny him the nomination?

(COMMERCIAL BREAK)

BLITZER: I've got it.

More now on the breaking news. President Obama endorsing Hillary Clinton for president only hours after a private meeting in the Oval Office with Bernie Sanders. The announcement coming in a video released by the Hillary Clinton campaign. Watch this.

(BEGIN VIDEO CLIP)

[18:31:03] BARACK OBAMA, PRESIDENT OF THE UNITED STATES: For more than a year now, across thousands of miles in all 50 states, tens of millions of Americans have made their voices heard. Today I just want to add mine.

I want to congratulate Hillary Clinton on making history as the presumptive Democratic nominee for president of the United States.

Look, I know how hard this job can be. That's why I know Hillary will be so good at it. In fact, I don't think there's ever been someone so qualified to hold this office. She's got the courage, the compassion, and the heart to get the job done. And I say that as somebody who had to debate her more than 20 times.

Even after our own hard-fought campaign, in a testament to her character, she agreed to serve our country as secretary of state. From the decision we made in the situation room to get bin Laden to our pursuit of diplomacy in capitals around the world, I have seen her judgment; I've seen her toughness; I've seen her commitment to our values up close. I've seen her determination to give every American a fair shot at opportunity, no matter how tough the fight was. That's what has always driven her, and it still does.

So I want those of you who have been with me from the beginning of this incredible journey to be the first to know that I'm with her. I am fired up, and I cannot wait to get out there and campaign for Hillary.

Sara, Secretary Clinton now has the support of the current president, the first lady, as well; expected to pick up endorsements soon from the vice president, Joe Biden; Senator Elizabeth Warren and her husband, who's part of that all-star campaign group of surrogates. She's going to have a lot going on for her right now. How will they be used specifically, though, by the campaign?

SARA MURRAY, CNN POLITICAL CORRESPONDENT: Well, that's absolutely right, Wolf, but it's a big contrast to what you're seeing on the Republican side, the fact that Hillary Clinton does have this deep bench of experienced surrogates who are willing to go out there in these battleground states and hammer home her message.

But it also sends a signal that the party really is uniting behind their nominee.

And this is the exact opposite of what we're seeing on the Republican side, Wolf, where we're seeing the leaders of the Republican Party either hold their noses and say that they're going to vote for or endorse Donald Trump, or as of this week, we're seeing people come out and say, "Look, I don't know if I'm ever going to be able to get behind this guy."

So it really sets up a contrast between, you know, how these parties are trying to come together, or not so much, after very contentious primaries.

BLITZER: Ryan, is this lineup equipped to fight this unconventional campaign by Donald Trump?

RYAN LIZZA, CNN POLITICAL COMMENTATOR: You know, I think that's -- that's a tough question to answer. I mean, so far no one has been able to figure out how to really go at Trump.

Look, he defeated what was considered by Republicans to be the 16 top Republicans in the party, right? Jeb Bush, Chris Christie, Scott Walker, Bobby Jindal, Ted Cruz. I mean, these were the creme de la creme of the Republican Party, and he dispatched with them with relative ease.

I would have said, you know, no, they don't know how to do this up until the speech she gave on foreign policy, where she really figured out how to get under Trump's skin and how to sort of put together all the anti-Trump lines in one way with some humor and some seriousness, and you know, that was sort of a turning point in the campaign. It obviously also came at the same point as his remarks about the judge, but it seems like they're finding their sea legs. And now one thing they still need to do is unite the Democratic Party and make sure that the Sanders base of the party gets behind her.

[18:35:00] BLITZER: Which raises my next question to you, Manu. President Obama, Senator Elizabeth Warren, they're both hugely popular with those Bernie Sanders supporters. So how will their endorsements help Secretary Clinton unify the party?

MANU RAJU, CNN SENIOR POLITICAL CORRESPONDENT: Well, clearly, they're going to be used to shore up her biggest vulnerability, which are younger voters, very progressive voters, those Bernie Sanders supporters, and people who say that they're Bernie or bust. Can Barack Obama, can Elizabeth Warren fire up that base? But you know, we know that -- how much have endorsements really mattered in this cycle? I mean, that's one school of thought. A lot of people are skeptical that it will mean much, Wolf.

But at the end of the day, I really agree with what Sara said earlier. This is such a contrast with the Republican side. I mean, the key thing for surrogates is to create that echo chamber. You know?

MURRAY: Right, and Trump doesn't have people to do that.

RAJU: Trump doesn't that. He doesn't have that. And when you can drill home and fire home one message from a variety of different people who have a big megaphone, that can have an effect.

But when Trump, you know, you notice in the last several days, what pushback has there been against Barack Obama, against coming out against -- you know, coming out for Hillary Clinton or anything else? There's just been nobody on the Republican side amplifying that message.

LIZZA: And if anything, there's a unified surrogate-like process of condemning Trump, of Republicans coordinating and saying, "Wait a second, for the sake of the party, we all have to, in unison, make sure that we're condemning those remarks." So it's -- I mean, that's very rare.

BLITZER: Jeffrey, President Obama will be campaigning, as we pointed out, next Wednesday with Secretary Clinton in Wisconsin. What type of role do you think he will be playing in the next month or so leading up to the Democratic convention in Philadelphia at the end of July? What role will the vice president, Joe Biden, play?

JEFFREY TOOBIN, CNN SENIOR LEGAL ANALYST: Well, I think those roles are -- have changed over the last six months, because the president has become a lot more popular than he was. He is now a lot more valuable in endorsement than he was six months ago.

I mean, think about the difference between three recent presidents, Ronald Reagan, George W. Bush and Clinton. George W. Bush and Bill Clinton were not succeeded by candidates of their own party. They were arguably repudiated by voters.

Barack Obama wants to be like Ronald Reagan, and he is at this moment about as popular as Ronald Reagan was at this point in his presidency. And just as Reagan was a tremendous asset to George Herbert Walker Bush, this looks like, at least at this point, that Barack Obama is going to be a real asset across the country for Hillary Clinton.

BLITZER: Fair point. All right, guys. Stand by. We have more to discuss. We're getting word that more top Republicans are now saying they're not yet ready, won't be able to support Donald Trump. Much more right after this.

(COMMERCIAL BREAK)

[18:42:35] BLITZER: Anxiety over Donald Trump is roiling the Republican Party right now, with some top officials keeping their presumptive presidential nominee at arm's length and others repudiating him outright.

Once again, let's bring back our panel.

Sara, John Kasich, he's the governor of Ohio. He said he could absolutely go to the convention and not endorse Donald Trump. And he saying, and I'm quoting now, "Why would I feel compelled to support someone whose positions I kind of fundamentally disagree with?"

What are the options for Republicans who feel that way?

MURRAY: Well, I think John Kasich is in a little bit of a more awkward position, because he is the governor of Ohio. The convention is in Cleveland.

And look, Donald Trump has done some outreach. He's called John Kasich. It was before Trump went after the judge, before Trump went after Susana Martinez, but it doesn't look like he has been able to patch things up.

And I think that we've already seen a number of Republicans say that they're going to skip this altogether. And I think we will see over the next couple of weeks whether Trump is willing to do this kind of outreach, these kind of one-on-one phone calls and try to sort of assuage concerns within the Republican Party. And for some people, like for John Kasich, that's just not going to be enough.

BLITZER: Jeffrey, the House Speaker, Paul Ryan, as you know, he's called Trump's attacks on the federal Judge Curiel distracting. He said he hopes Trump can run a campaign that people can be proud of. Is this all, though, setting a bad example right now? How do they get out of this?

TOOBIN: It's really a hard problem. I mean, think about this. This has never happened in the modern era, where you have the presumptive nominee of the party who is not supported by a very significant part of that party.

You have Jeb Bush. You have the previous nominee for president, Mitt Romney, who's called him a con man, who clearly is not -- is not going to support him.

But I think it is either going to be a disaster or we're going to be in such a new world where the party is essentially irrelevant to the candidate, but that's a very tough sell when, you know, we do live in a two-party system, and one party usually is unified against the other unified party.

BLITZER: Ryan, you just reported that a top Republican senator isn't even necessarily ruling out the possibility of voting for Secretary Clinton, as opposed to Donald Trump. What can you tell us?

LIZZA: Yes, Senator Susan Collins of Maine, long time moderate Republican, in an interview yesterday with me was talking about this awful choice that she has before her, and talking about how Trump's comments about Judge Curiel were order of magnitude worse than all of the other things that had concerned her about Trump.

[18:45:14] Now, interestingly, she had never endorsed Trump and she still hasn't but she made the point -- she pointed out she has a good working relationship with Senator Clinton, when Clinton was in the Senate and when Clinton was secretary of state and she said it was unlikely but that she would not rule out the possibility of backing Hillary Clinton over Donald Trump. That's very unusual. You know, even the most anti-Trump, Never Trump folks, like Senator Ben Sasse of Nebraska or Mitt -- even Mitt Romney during the primaries, they've said they could never, ever support Hillary Clinton.

So, Susan Collins is in a very different place, she's saying it is unlikely she would support Clinton but she will not rule it out. As far as I know, that's the only senior Republican who is in that camp.

BLITZER: That's fascinating. Ryan doing good reporting for us.

Manu, even among Republicans who say they'll vote for Donald Trump, they're making a clear distinction that they aren't ready to endorse him. They said they'll vote for him but they won't endorse him. What does that mean for Trump's general election campaign? RAJU: Well, it means what Jeffrey said earlier, that a large segment

of the Republican Party, particularly in Washington, is not ready to get behind Donald Trump. I spent this week actually talking to a lot of vulnerable senators in difficult reelection races. Some of them are OK with endorsing Donald Trump, others not so much, including Kelly Ayotte who made the distinction she was vote for Donald Trump but she's not ready to endorse him.

And same with Ron Johnson said the same thing. "I'm going to vote for him, not ready to endorse him."

I don't know what the distinction is between that, but they're trying to at least show, have some -- to say "we're not endorsing and embracing all of Donald Trump's policies."

But I will say one other thing, Jeffrey, Ron Johnson, it's interesting, because he -- several months, several weeks ago, during primary season, I was out in Wisconsin with him and he said Donald Trump would be good for him, said I would stump with him, like Ronald and the Donald stumping together. But now, he doesn't want to say I will endorse him. So, a shift and a concern among vulnerable Republicans.

TOOBIN: But I just think also, it is worth pausing to -- I mean, I consider myself fluent in the English language, I know all the words. The difference between support, endorse, vote for, they're all the same as far as I can tell. I mean, the idea that politicians are drawing distinctions where -- you know, English means what English means.

(CROSSTALK)

BLITZER: I think Jeffrey --

LIZZA: Go ahead, Wolf.

BLITZER: I think the point they're trying to make is they'll vote for Donald Trump, but you won't see them on the campaign trail actively campaigning for Donald $ump. That may be the distinction they're trying to suggest.

Guys, stand by. We have more coming up, much more news right after this.

(COMMERCIAL BREAK)

[18:52:31] BLITZER: Brazil is scrambling right now to prepare for the Summer Olympic Games, but their serious concern, Rio won't be ready in time.

Our senior international correspondent Nick Paton Walsh is there.

(BEGIN VIDEOTAPE)

NICK PATON WALSH, CNN SENIOR INTERNATIONAL CORRESPONDENT (voice-over): Rio has a big question without an answer ready just yet. How do Olympic tourists get from their hotels here to the games across town? Without spending hours in this, some of the worst traffic in South America.

Well, this was meant to be the answer. An extension to the subway from the beaches almost to the Olympic park. But there's just one snag. They've just announced a new updated opening time, and that's only four days before the games begin.

(on camera): It's always going to be some sort of last-minute rush. But it's a sheer amount of political and economic upheaval that Brazil is experiencing that's got many concerned that leaving such a vital part of the infrastructure as this down to last-minute preparations is simply cutting it too fine.

(voice-over): It was meant to be open in July. And without it, guests may spend a lot of the day in jams. That's not going to happen, insists the government.

RODRIGO VIEIRA, TRANSPORTATION SECRETARY: We are completely sure that everything will be done. No problem for us. Of course, the schedule is tight. But we have 8,000 people working during the days and during the nights. No problem at all.

WALSH (on camera): The sound of building is so loud, it's drowning you out. So, we still have quite a bit more time to go, right, until this is ready.

VIEIRA: Everything is in our schedule.

WALSH (voice-over): They said the same thing about the Olympic Park itself. But when we visited when there were 66 days to go, it didn't feel that ready.

(on camera): It's strange to be able to walk straight in from there, right into the edge of the Olympic Park here. What's supposed to be a pretty secure zone in just a matter of weeks from now, and we're just going to walk down this way to the site of where previously there was one man holding out with his home.

(voice-over): Deeper and deeper, we went, security sitting by to find the home now demolished, the owner taking a payout and moving. An odd feeling, walking so freely around.

This worker told us sometimes security are there and some days, they're not.

Living just alongside and refusing to be moved are Sandra and Maria.

[18:55:05] They call themselves the resistance and they force authorities to accept they can stay on the land. Sandra says she'll soon have all this packed away, ready for the new home the city is building her just next door. That's also on a tight schedule. Supposed to be ready, she says, 12 days before the games begin.

What does Maria think about security? MARIA DA PENHA, RESIDENT (through translator): It should be like that

in every country. We were born to walk freely. They came up with so much security. A man doesn't make another one safe. Security comes from God.

WALSH: You have to hope they won't be leaving it just up to Him, however, to get Rio ready in time.

Nick Paton Walsh, CNN, Rio de Janeiro, Brazil.

(END VIDEOTAPE)

BLITZER: Another huge concern for the Olympics, the Zika virus, spread by mosquitoes and known to cause severe birth defects. Some Olympic athletes say they won't take part in the games because of the virus.

Let's dig deeper into this with the director of the National Institute of Allergy and Infectious Diseases, Dr. Anthony Fauci.

Dr. Fauci, thanks for joining us.

First of all, is the U.S. government doing enough right now to research and fight the Zika virus?

DR. ANTHONY FAUCI, NATIONAL INSTITUTE OF ALLERGY AND INFECTIOUS DISEASE: Well, we certainly are. As you know, we have an issue of making sure that we get the proper funding to do it. But the NIH and CDC are with money that we have taken from other funds, going ahead right now. And doing things such as mosquito vector control, which the CDC and their staff are very, very much involved with, and helping the individuals and the agencies in South America, the Caribbean, particularly Puerto Rico, but Brazil also.

And we are in the process of developing a vaccine, Wolf, that we hope will get into the initial studies to determine if it's safe by hopefully, and I believe it will meet that landmark that we will be there in September of 2016 to get that at least started to determine if it's safe. And then we'll go into a much bigger efficacy study as we get into 2017. So, we are really pushing the envelope, both from the CDC standpoint and from the NIH standpoint.

BLITZER: You have been the director over there at the infectious disease center since, what, 1984, a time when HIV/AIDS was just beginning to emerge in the United States. Let me play a short clip for you, a news reel clip from the CNN original series "THE EIGHTIES", which documents the fight against AIDS during that decade.

Listen to this.

(BEGIN VIDEO CLIP)

TV ANCHOR: Scientists at the National Centers for Disease Control in Atlanta today released the results of a study which shows that the lifestyle of some male homosexuals has triggered an epidemic. REPORTER: Bobbie Campbell of San Francisco and Billy Walker of New

York both suffer from a mysterious newly discovered disease which affects mostly homosexual men.

UNIDENTIFIED MALE: Our best guess is that it's somehow related to gay lifestyle.

UNIDENTIFIED MALE: I was in the fast lane at one time in terms of the way I lived my life and now I'm not.

REPORTER: Researchers know of 413 people who have contracted the condition in the past year. One-third have died and none have been cured.

(END VIDEO CLIP)

BLITZER: Tell us about the difficulties of identifying and fighting a disease that was so poorly understood at the time back in the '80s.

FAUCI: That was an extraordinary time, Wolf. I, as you know, got involved with that literally right from the very first reports that came from the CDC in the summer of 1981, and it was very mysterious and very frightening. We were dealing with young men, almost all gay men who were desperately ill with an illness that we had no idea what the cause was.

Many of us from the beginning suspected that this was a viral disease, but we did not know what it was, because it was a virus that we had never seen before. And it really wreaked havoc mostly in the gay community, with a lot of suffering and in the beginning, there was a lot of inertia in getting things moving and calling attention to this. And there was just handfuls of us that were involved at the CDC, at the NIH, and some of the physicians in California, San Francisco, L.A. and in New York.

And then it became clear that this was something that was just the tip of the iceberg. And when the virus was recognized and we had a diagnostic test, it was stunning, Wolf, how many people were already infected at the time that we knew that we were dealing with a new disease.

BLITZER: Really amazing. And I want to thank you on behalf of everyone for all the incredibly important work you have done over all of these decades. Dr. Fauci, thanks very much for joining us.

An important note to you're viewers, "The AIDS Crisis", "THE EIGHTIES", airs later tonight, 9:00 p.m. Eastern and 9:00 Pacific, right here on CNN. You will want to see that.

Thanks very much for joining us. I'm Wolf Blitzer in THE SITUATION ROOM.